1. Field of the Invention
The present invention relates to a drive circuit for generating a driving waveform corresponding to brightness data; a display device therewith; a driving method for generating the driving waveform; and more specifically to a method of driving a light-emitting device in an image display device provided with an image display panel having the matrix wiring of a plurality of light-emitting devices.
2. Related Background Art
Up to now, two kinds of electron emission devices, that is, a hot cathode device and a cold cathode device are known. Among these, as a cold cathode device, for example, a surface conduction electron-emitting device, a field emission type device (hereafter, an FE type device), a metal/insulating film/metal type discharge device (hereafter, an MIM type device), etc. are known. As a surface conduction electron-emitting device, for example, a device disclosed in an article of “M. I. Elinson, Radio Eng., Electron Phys., 10, 1290 (1965)”, and other examples described later are known.
A surface conduction electron-emitting device uses a phenomenon that electron emission occurring by letting a current in a thin film with a small area, which is formed on a substrate, in parallel with a film surface. As this surface conduction electron-emitting device, besides the device by Elinson et al. where an SnO2 thin film is used, a device consisting of an Au thin film (G. Dittmer: Thin Solid Films, 9, 317 (1972)), a device consisting of In2O3/SnO2 thin film (M. Hartwell and C. G. Fonstad: IEEE Trans. ED Conf., 519 (1975)), a device consisting of a carbon thin film (Hisashi Araki, et al.: Vacuum, 26th volume, No. 1, 22 (1983)), and the like were reported.
As a typical example of the device structure of these surface conduction electron-emitting devices, a plan of the above-mentioned device by M. Hartwell et al. is shown in FIG. 28. In the figure, reference numeral 3001 denotes a substrate and numeral 3004 denotes an electro conductive thin film made of metallic oxide formed by sputtering. The electro conductive thin film 3004 is formed in H-shaped plane geometry as shown in the figure. An electron emission part 3005 is formed by performing the energization processing which is called below-mentioned energization forming, to this electro conductive thin film 3004. A gap L in the figure is set within 0.5 and 1 mm, and w is set at 0.1 mm. In addition, although the electron emission unit 3005 is shown in rectangular geometry in the center of the electro conductive thin film 3004 from convenience of illustration, this is schematic and is not necessarily expressing the location or geometry of an actual electron emission unit faithfully.
In the above-described surface conduction electron-emitting devices including the device by M. Hartwell et al., it is common to form the electron emission unit 3005 by performing the energization processing, called energization forming, to the electro conductive thin film 3004 before performing electron emission. Namely, the energization forming means to form the electron emission unit 3005 in a highly resistive state electrically by applying a fixed DC voltage or, for example, a DC voltage, which increases at a very slow rate which is about 1 V/min, to both ends of the electro conductive thin film 3004, to locally break or deform the electro conductive thin film 3004, or to change its quality. In addition, a crack arises in a portion of the electro conductive thin film 3004 which is locally broken, deformed or changed in quality. When a proper voltage is applied to the electro conductive thin film 3004 after the above-described energization forming, electron emission occurs near the above-described crack.
As examples of FE type devices, for example, devices reported by the articles of “W. P. Dyke & W. W. Dolan, Field emission, Advance in Electron Physics, 8, 89 (1956)”, and “C. A. Spindt, Physical properties of thin film field emission cathodes with molybdenum cones, J. Appl. Phys., 47, 5248 (1976)” are known.
As a typical example of device structure of an FE type, a sectional view of the above-mentioned device by C. A. Spindt et al. is shown in FIG. 29. In this figure, reference numeral 3010 denotes a substrate, numeral 3011 does emitter wiring made of conductive material, numeral 3012 does an emitter cone, numeral 3013 does an insulating layer, and numeral 3014 does a gate electrode. This device makes field emission occur from an end portion of the emitter cone 3012 by applying a proper voltage between the emitter cone 3012 and gate electrode 3014. In addition, as another device structure of the FE type device, there is also an example of arranging an emitter and gate electrodes nearly in parallel with a substrate plane on a substrate except the laminated structure as shown in FIG. 29.
As an example of an MIM type device, for example, a device reported in an article of “C. A. Mead, Operation of tunnel emission Devices, and J. Appl. Phys., 32, 646 (1961)” is known. A typical example of the device structure of an MIM type device is shown in FIG. 30. This figure is a sectional view, and in the figure, reference numeral 3020 denotes a substrate, numeral 3021 does a lower electrode made of metal, numeral 3022 does a thin insulating layer with the thickness of about 100 Å, and numeral 3023 does an upper electrode made of metal with the thickness of about 80 to 300 Å. In the MIM type device, electron emission is made to occur from a surface of the upper electrode 3023 by applying a proper voltage between the upper electrode 3023 and lower electrode 3021.
Since the above-described cold cathode device can obtain electron emission at low temperature in comparison with a hot cathode device, it does not need a heater for heating. Hence, since its structure is simpler than that of a hot cathode device, it is possible to produce a fine device. In addition, even if plenty of devices are arranged in high density on a substrate, it is seldom to generate problems such as a thermofusion of a substrate. Moreover, differently from slow response speed of a hot cathode device due to an action by the heating of a heater, the cold cathode device also has an advantage that response speed is quick. For this reason, researches for applying a cold cathode device have been done actively.
For example, a surface conduction electron-emitting device has an advantage that plenty of devices can be formed over a large area since the surface conduction electron-emitting device is simple in structure and is easily produced. Then, as disclosed in, for example, Japanese Patent Application Laid-Open No. 64-31332 applied by the present applicant, methods for arranging and driving many devices have been studied. In addition, as for the application of surface conduction electron-emitting devices, image formation apparatuses such as an image display unit and an image recording device, a source of a charged beam, and the like have been studied.
In particular, as for the application to image display units, as disclosed in, for example, U.S. Pat. No. 5,066,883, Japanese Patent Application Laid-Open No. 2-257551, Japanese Patent Application Laid-Open No. 4-28137, and the like, image display units where a surface conduction electron-emitting device and phosphor which emits light by irradiation of an electron beam are combined and used have been studied. The image display units where a surface conduction electron-emitting device and phosphor are combined and used are expected in characteristics superior to those of conventional image display units where other methods are used. For example, even if it is compared with an LCD which has spread in recent years, it can be said that it is excellent in terms of not requiring a backlight since it is a spontaneous light type unit, and in terms of a wide viewing angle.
In addition, a method of arranging and driving plenty of FE type devices is disclosed in U.S. Pat. No. 4,904,895. In addition, as an example of applying an FE type device to an image display unit, for example, a flat plate type display unit reported by R. Meyer et al. is known (R. Meyer: Recent Development on Microtips Display at LETI, Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)).
In addition, as an example of applying plenty of MIM type devices to an image display unit is disclosed in Japanese Patent Application Laid-Open No. 3-55738. Furthermore, a unit where an EL (electroluminescence) device is used is disclosed in, for example, Japanese Patent Application Laid-Open No. 09-281928 as an image display unit where a device other than an electron emission device is used.
The present inventor et al. has tried, for example, a multi-electron beam source by an electric wiring method shown in FIG. 31. Thus, it is a multi-electron beam source where plenty of electron emission devices are arranged two-dimensionally, and are wired in a matrix as shown in the figure.
In the figure, reference numeral 1 schematically denotes an electron emission device, numeral 2 does row-directional wiring, and numeral 3 does column-directional wiring. The row-directional wiring 2 and the column-directional wiring 3 have wiring resistance 4 and 5, wiring inductance 6 and 7, and wiring capacitance 8. In addition, although the device is shown in a 4×4 matrix for the convenience of illustration, of course, the scale of the matrix is not necessarily restricted to this, but in the case of, for example, a multi-electron beam source for an image display unit, a sufficient number of devices for performing desired image display are arranged and wired.
In a multi-electron beam source where the matrix wiring of electron emission devices is performed, proper electric signals are applied to the wirings in the row and column directions so as to make a desired electron beam output.
A pulse width modulation waveform is shown in FIG. 32. For example, so as to drive electron emission devices in an arbitrary row in a matrix, selection potential Vs is applied to the wiring in the direction of a row selected, and non-selective potential Vns is simultaneously applied to the row-directional wirings not selected. Drive potential Ve for outputting an electron beam is applied to column-directional wirings in synchronizing with this. According to this method, a voltage of Ve−Vs is applied to the electron emission devices in the row selected, and a voltage of Ve−Vns is applied to the electron emission devices in the non-selective rows. An electron beam with desired intensity is outputted only from an electron emission device in a selected row if Ve, Vs, and Vns are made to be proper potential. In addition, since the response speed of a cold cathode device is high, if the length of time for applying drive potential Ve is changed, it is possible to change the length of time when the electron beam is outputted. Similarly, it is possible to control an electron beam also by a method which is called level modulation and which controls luminance brightness by changing potentials and current values which are applied to the column-directional wirings.
By the way, in a display unit having the effective pixel count of 1920×1080, a frame rate of 60 Hz, and 10-bit gradation, in the case of a pulse level modulation, in letting a level of energy, applied to a device, be Pi, the resolution of Pi/210=Pi/1024 is needed. In voltage drive, since pi becomes several volts, the resolution of several millivolts is required in a driving waveform over the whole screen of 1920×1080 pixels. It is difficult to realize this value when considering characteristics of an IC, a printed circuit board, and a power supply which constitute a drive circuit.
On the other hand, in the case of a pulse width modulation, time for driving one scanning line is 1/(60×1080)≈15 μsec. When 10-bit pulse width modulation is performed, minimum pulse width is 1/(60×1080×210)≈15 ns, and hence, the minimum pulse width resolution of 15 ns is needed.
However, wiring shown in FIG. 31 is equivalent to a low-pass filter with a cut-off frequency determined by wiring inductance (L), wiring capacitance (c), and wiring resistance (R). When signal wiring and display wiring which have such low-pass characteristics are driven by a line sequential-pulse width modulation (PWM) driving system consisting of frequency spectrum components higher than a cut-off frequency, as shown in FIG. 33, leading and trailing waveform of a PWM waveform which is applied to a device become dull, and hence, display quality in low luminance brightness is degraded. In particular, a synthetic waveform with an output waveform of a scan circuit 11 which is applied to the electron emission device 1 becomes a waveform whose level becomes low when the pulse width modulation driving waveform at low gradation is applied from an information electrode drive circuit 10. That is, since a level of a driving waveform which consists of only high frequency spectrum components, that is, a pulse width modulation driving waveform at low gradation becomes low, it is not possible to display an image at desired gradation in a low gradation region.
In addition, also when a constant current pulse with short time length is supplied from a control constant current source to a multi-electron source where great many electron emission devices are wired in a matrix, electrons are hardly emitted. When a constant current pulse is supplied for a comparatively long period, of course, electrons begin to be emitted, but long leading time was needed until electron emission began.
FIG. 33 is a time chart for explaining this, and as shown in the figure, even if a control constant current source supplies a short current pulse, a current If hardly flows into an electron emission device. In addition, even when a long pulse is supplied, the drive current If which flows into an electron emission device becomes a waveform with large leading time. Although a cold cathode type electron emission device itself has high-speed responding capability, a current waveform supplied to the electron emission device becomes dull, and hence, a waveform of an emission current Ie is also deformed as a result.
In a multi-electron source where simple matrix wiring is performed, as the scale of a matrix is enlarged, parasitic capacitance (wiring capacity) increases in connection with it. Main portions of parasitic capacitors exist in intersections of row-directional wiring and column-directional wiring, and this equivalent circuit is shown in FIG. 34. When a control constant current source 9 connected to column-directional wiring 3 starts to supply a constant current Il, the current is spent for charging a parasitic capacitor 8 in a starting stage not to serve as a drive current of the electron emission device 1. For this reason, the effective response speed of the electron emission device falls.
In addition, as for voltage drive, there are the following troubles to be solved. Generally, on a display unit using a device where a current flows with drive as a light emitting device, for example, LED, EL, FED, SED, etc., wiring resistance is designed to be low. Hence, its equivalent circuit is a model which is shown in FIG. 31 and is constituted by parasitic capacitance, parasitism resistance, and parasitism inductance. If a conventional voltage driving method is applied to such a circuit, since a charging current i flows into a parasitic capacitance by the application of a voltage, a leading edge of a driving waveform becomes dull. Furthermore, by a self-induction action of the parasitism inductance, electromotive force U=−Lx(di/dt) arises, overshoot and ringing arise, and the application of an abnormal voltage to a light emitting device arises.
In recent years, demand for display units with a large area, high resolution, and fine gradation has been remarkable, parasitic inductance and parasitic capacitance of wiring have increased in connection with it, and hence, elimination of gradations in a low luminance brightness region which is caused by dullness, an overshoot, and ringing of a leading edge of a driving waveform have become increasingly important problems to be solved.
In addition, it has become a problem that it becomes impossible that a driving waveform by simple pulse width control and pulse height value control guarantees the monotonicity of gradation because of changes and dispersion of voltage/luminescence intensity characteristics of light emitting devices.
In addition, for example, as disclosed in Japanese Patent Application Laid-Open No. 09-319327, a method and the like have been performed, the method in which a charge voltage is applied in addition to a drive current pulse by a control current source for supplying a drive current pulse to the above-described cold cathode device, a voltage source for charging parasitic capacitors of a multi-electron source at high speed, and charge voltage application means of electrically connecting the above-described voltage source with the above-described column-directional wiring in synchronizing it with an leading edge of the above-described drive current pulse, until charging to the parasitic capacitance of wiring is almost completed. When such drive is performed, it becomes possible to guarantee the linearity of gradation.
In addition, in Japanese Patent Application Laid-Open No. 8-22261, a driving waveform which has a period longer than a period of a time slot of a conventional PWM waveform is realized by dividing each word of a digital image signal into a plurality of sub words and assigning a PWM waveform, whose level is low, to a lower sub word, and a PWM waveform, whose level is high, to a higher sub word, and the deterioration of image display quality in low luminance brightness is prevented.
In addition, in Japanese Patent Application No. 10-39825, a problem of necessity of frequency increase of a PWM operating frequency which poses a problem with an increase of gradations is solved by making it possible to reduce a frequency in a pulse width modulation circuit with a drive method of having second pulse width modulation output means of outputting a binary signal whose high and low voltages are V1 and V2 respectively according to a luminance signal, and second pulse width signal output means of cutting the above-described binary signal in predetermined pulse width according to the above-described luminance signal.
Furthermore, in Japanese Patent Application No. 11-015430, fine gradation is easily realized by using a pulse driving waveform including information on M×N gradations, defined by pulse width control corresponding to M gradations, and pulse height value control corresponding to N gradations, as a voltage pulse.
However, in the drive by the conventional pulse width modulation, there is a further possibility of inducing large electromagnetic wave noise, i.e., the spurious radiation of an electromagnetic wave at leading and trailing edges of a driving waveform depending on gradation.
In addition, in a multi-electron beam source where many electron emission devices described above are arranged in a matrix, there is a problem that a voltage applied to each device becomes smaller as the device is apart from its feeding terminal due to a voltage drop caused by an influence of its wiring resistance, and in consequence, the discharge electron distribution of each device does not become uniform. Then, when this multi-electron emission device is applied to an image display unit, there is a problem that image quality deteriorates due to a voltage drop caused by a wiring resistor.
This will be described by using FIGS. 34 and 35. FIG. 34 shows an example of a substrate of a multi-electron beam source. In the figure, reference numeral 1 denotes an electron emission device, numeral 2 does a selection electrode (row-directional wiring), numeral 3 does an information electrode (column-directional wiring), numeral 9 does a selection circuit, numeral 10 does a modulation circuit, and numeral 12 does the substrate.
In addition, FIG. 35 is a perspective view of an image display panel where the substrate 11 of a multi-electron beam source shown in FIG. 34 is used. In the figure, reference numeral 13 denotes a metal back, numeral 14 does a fluorescent screen, numeral 15 does a faceplate, and numeral 16 does a current from an electron source.
Now, it is assumed that a certain selection electrode 2 is selected and all the pixels connected to the selection electrode lit up. An equivalent circuit at this time is shown in FIG. 36. In the figure, reference numeral 16 denotes a current component which flows from an information electrode to the selection electrode through an electron emission device, and numeral 4 does a resistive component of the selection electrode.
A current flowing into the selection electrode to each device is made into the same value If, and it is assumed that the resistance of a selection electrode per pixel is rf. Potential on the selection electrode at this time is calculated.
A current which flows into Rf5 is If, and an amount of a voltage drop by Rf5 is If·rf. A current which flows into Rf4 is 2·If, and an amount of a voltage drop by Rf4 is 2·IF·rf. Similarly, an amount of a voltage drop in each resistive component is calculated, and the result of calculating the potential of each portion on the selection electrode is shown in FIG. 37. In addition, here, the case of Ve>Vs is shown.
A remarkable point is that potential rises as a place is apart from a feeding point since currents flow into the selection electrode 2 when potential Vs is outputted from the selection circuitry 9 which is the feeding point, and the potential rises at the most distant edge by 21·If·rf. FIGS. 38A, 38B and 38C show driving waveforms applied to a pixel in the most distant edge at this time. In the figure, FIG. 38A shows a potential waveform applied to a selection electrode, FIG. 38B shows a potential waveform applied to an information electrode, and FIG. 38C shows a voltage waveform applied to the selected electron emission device. It can be seen that a voltage applied to the device falls because selection potential becomes Vs′ from Vs.
Although this voltage dispersion does not pose a problem so much when a resistive component of a selection electrode is very small, for example, if the resistive component of a selection electrode is large due to an increase of screen size of an image display unit etc., the dispersion of the voltage cannot be disregarded. In addition, when a pixel count increases and the current which flows into a selection electrode increases, the voltage dispersion becomes large.
When this voltage dispersion arises, a voltage applied to an electron emission device differs every device, and in particular, an electron emission device near a feeding point and an electron emission device which is apart from the feeding point are not given the same voltage, and hence, difference arises in the amount of electron emission. This appears as the difference of luminance brightness between pixels which are elements which emit light by an electron beam emitted from its electron emission device, and leads to the degradation of display quality as an image display unit.
It is disclosed in Japanese Patent Application Laid-Open No. 10-112391 to make plenty of light emitting devices emit light uniformly, and to realize excellent characteristics as an image display unit by paying attention to the resistance of a wiring electrode and a current flowing in the wiring electrode in an X-Y matrix type organic EL display unit, adopting a drive method of performing driving with a current source connected to a voltage source with a drive voltage of Vcc while providing a data electrode in low resistance wiring and a scan electrode in high resistance wiring, and making the drive voltage Vcc at this time be equal to or more than a specific voltage satisfying conditions under which the current source surely performs constant current operation even if there is dispersion in wiring resistance depending on a location of a light emitting device which is a pixel.
In addition, it is mentioned in Japanese Patent No. 3049061 to divide a trailing edge of a signal, applied to modulation wiring (information signal wiring), into a plurality of steps. In addition, in Japanese Patent Application Laid-Open No. 7-181917, a method is mentioned, the method which is for generating a driving waveform by using two or more voltages corresponding to a singular or plural number of unit drive blocks and stacking these unit drive blocks in the pulse width and level directions.